“Mn-locking” effect by anionic coordination manipulation stabilizing Mn-rich phosphate cathodes

High-voltage cathodes with high power and stable cyclability are needed for high-performance sodium-ion batteries. However, the low kinetics and inferior capacity retention from structural instability impede the development of Mn-rich phosphate cathodes. Here, we propose light-weight fluorine (F) doping strategy to decrease the energy gap to 0.22 eV from 1.52 eV and trigger a “Mn-locking” effect—to strengthen the adjacent chemical bonding around Mn as confirmed by density functional theory calculations, which ensure the optimized Mn ligand framework, suppressed Mn dissolution, improved structural stability and enhanced electronic conductivity. The combination of in situ and ex situ techniques determine that the F dopant has no influence on the Na+ storage mechanisms. As a result, an outstanding rate performance up to 40C and an improved cycling stability (1000 cycles at 20C) are achieved. This work presents an effective and widely available light-weight anion doping strategy for high-performance polyanionic cathodes.

dependent GGA plus Hubbard correction U (GGA + U) method, 7 and the effective U eff parameters are 3.9 and 3.7 eV for Mn-3d and Cr-3d states, 8 respectively.

Na 4 MnCr(PO 4 ) 3 is built from Na 4 Mn(PO 4 ) 3 with equal molar of Mn-Cr mixing, that is
Mn-Cr cation arrangements in the Mn sites were created within the conventional cell of Na 4 Mn(PO 4 ) 3 using the enumeration method 9 implemented in the Pymatgen code. 10 The Mn-Cr cation arrangement of the most stable Na 4 MnCr(PO 4 ) 3 structure was determined by ranking the Mn-Cr cation arrangements by their DFT energies.
Furthermore, determination of the stable structures of the sodium extracted compounds with Na-vacancy mixing, including Na 3 MnCr(PO 4 ) 3 , Na 2 MnCr(PO 4 ) 3 and NaMnCr(PO 4 ) 3 , was employed through this same method. Similarly, the most stable Fdoped Na 3.85 MnCr(PO 3.95 F 0.05 ) 3  discharging of an alkali-ion battery, an alkali A is intercalated or deintercalated from a host crystal structure A n H. For a battery that operates by shuttling x A + ions between the cathode and a pure alkali metal anode, the overall cell reaction can be written as follows: where the typical phase of each compound is indicated for clarity. The forward reaction is the cell discharging reaction, while the reverse is the cell charging reaction.
The average intercalation potential V vs. A/A + can then be calculated using the following expression: 12 where E is the total energy as calculated using DFT, and e is the absolute value of the electron charge.                 The D Na+ from CV tests can be calculated based on the Randles-Sevcik equation: [10][11] (Equation S2) = 2.69 × 10 5 3/2 A 1/2 C

1/2
Where i p refers to the peak current, n (2 in this test) is the amounts of transferred electrons, A (0.785 cm 2 ) is the surface area of electrode, C represents the Na + molar concentration (2.66 × 10 -2 mol cm -3 ), and v refers to the scan rates.  We further employed various techniques to characterize the commercial hard carbon material. As seen in Fig. S29a, the XRD pattern presents two broad characteristic peaks located at around 22° and 44°, which can be indexed to (002) and (100) peaks, respectively. The Raman spectrum displays two obvious peaks centered at ~1341.8 cm -1 (D band) and ~1575.7 cm -1 (G band), which are assigned to the A 1g symmetry vibration mode (the sp 2 carbon atoms of disordered graphite) and the E 2g symmetry vibration mode of sp 2 carbon atoms in chains and rings, separately. The intensity ratio of D band and G band (I D /I G ) is measured to be 1.14, indicating it possesses many defects. Besides, Fig. S29c shows four fitted peaks at 284.8 eV, 285.1 eV, 286.2 eV, and 290 eV that can be assigned to be sp 2 , sp 3 , C-O, and C=O, respectively. Fig. S29d-e present irregular morphologies of hard carbon materials with different particle sizes and relatively smooth surface (inset of Fig. S29d) and uniform elemental distribution.